Method for producing pyramidal shaped tumbling media



Dee. 22, 1970 J. B. KITTREDGE ETAL" PYRAMIDAL SHAPED TUMBLING MEDIA v Filed Aug. 5, 1968 I NVENTORS Jam/5. Mfr/e506:

5y Guy/5. LANGLO/S United States Patent 0 3,549,341 METHOD FOR PRODUCING PYRAMIDAL SHAPED TUMBLING MEDIA John B. Kittredge, White Bear Lake, Minn., and Guy B.

Langlois, Huntington Beach, Calif., assignors to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware Continuation-impart of application Ser. No. 487,721, Sept. 16, 1965. This application Aug. 5, 1968, Ser. No. 769,769

Int. Cl. B24d 3/14; C08g 51/12 US. Cl. 51-293 4 Claims ABSTRACT OF THE DISCLOSURE Conical or pyramidal shaped ceramic or polymeric tumbling media having height about 0.75 to 1.5 times diameter formed by dropwise feeding from an orifice of a thixotropic composition having a viscosity between 500 and 30,000 poise, measured on a rotating spindle viscometer at 2 r.p.m., and yield stress value of at least 15,000 dynes/cmfi, depositing droplets on a substantially flat surface to form upwardly tapering shapes and hardening shape to withstand a crushing force of at least 50 p.s.i.

This application is a continuation-in-part of our copending application Ser. No. 487,721, filed Sept. 16, 1965, and now abandoned.

This invention relates to tumbling media for industrial finishing and a method for the formation thereof. More particularly the invention relates to tumbling media comprising shapes of relatively uniform size and shape containing abrasive particles and a binding material.

The mechanical finishing industry utilizes a wide variety of media or shapes for the surface refinement and/or deburring of metal and plastic parts. These media have included natural or synthetic stone, porcelains, abrasive filled clays, wood, leather, plastics and the like. The parts to be refined or deburred are placed in a tumbling drum together with the media which support the parts, prevent undue collision thereof and carry or supply the abrasive which performs the deburring or cutting. Irregularly shaped media such as natural stones can cause severe production problems because of lodgingthe tendency of the shapes to jam into a slot, hole or undercut and resist removalover a wide range of slot dimensions. Accordingly efforts have been made to provide media in more precise uniform shapes.

Methods which have been used for forming suitable media involve the casting of a slip or other casting composition into a plurality of cavities, dry pressing a powder abrasive-containing composition, or injection molding pastelike compositions. Each of these techniques entails a large number of molds which, in addition to their high cost, result in mold release problems, and/or require elaborate high speed equipment in which wear problems are high. Another method which is used involves extrusion and cutting to length of a suitable pastelike composition. This procedure also entails the use of costly molding and cutting equipment. These problems are magnified by the large number of uniform pieces required. The individual pieces commonly weigh between 1 and 10 grams and are of such size that 2500 to 75,000 pieces per cubic foot may be required. Industrial mechanical finishing barrels or vibrators commonly range in size from 2 to cubic feet. The manufacture of sharp pointed tetrahedrons and cones has been particularly costly and diflicult.

The present invention has made possible the formation of finishing media very uniform in size, at high production rates, and at a cost significantly less than methods previously used. It has been discovered that by 3,549,341 Patented Dec. 22, 1970 using mixtures in a viscosity range intermediate to those previously used for either extrusion or casting that shapestable generally conical shapes can be formed by feeding the composition dropwise out of an orifice or preferably a plurality thereof onto a suitable substantially flat surface such as an endless belt. Furthermore the invention has made possible the economical formation at high production rates on relatively simple apparatus of tumbling media in essentially sharp-pointed conical or polyhedral shapes.

The present invention makes possible the tailoring of a Wide range of products from those designed for rapid, coarse cutting to intermediate finishing to fine polishing. The finishing characteristics of the media are easily controlled by variation of the hardness, amount and size of filler particles, the media size, and the hardness of the binder composition.

The media of this invention comprises a plurality of shapes each having a flat cast surface, the other surfaces of each shape being smoothly tapered together toward a point remote from the flat surface, the ratio of the distance between the point and flat surface to the diameter of the flat surface being between about 0.75 and 1.5. Conical, tetrahedral, and pyramidal shapes are representative of those which can be produced in accordance with the invention. The shapes of this invention provide a plurality of working edges and surfaces which can penetrate into recesses or undercut portions of the parts being worked and which are free of parallel surfaces or gradually tapered contours which would tend to lodge in the parts being finished.

Briefly summarized, the process of this invention includes the steps of providing a thixotropic flowable, permanently settable composition 'which has a viscosity between about 500 and 30,000 poise (measured on a rotating spindle viscometer at 2 r.p.m.), the thixotropy of the composition being sufiicient to maintain a droplet thereof supported on a flat surface at at least a height of 0.75 times the diameter of the droplet. It has been found that most thin, fiowable liquids which could be pumped through an orifice cannot be used because they would slump over into a flat button shape when deposited or dropped onto a receiving surface. On the other hand, many viscous materials Which would form slump resistant shapes cannot be pumped out of an orifice and deposited as separate droplets because their viscosity is too high. In accordance with the present invention, these problems are overcome by the use of a thixotropic material having a yield stress of at least 15,000 dynes/cm. as measured on a Ferranti- Shirley plate and cone viscometer. Liquids having this characteristic in addition to the above defined viscosity characteristic are both fiowable through an orifice, separable under gravity into individual droplets, and capable upon deposit by dropping onto a fiat surface of maintaining a shape having a height of at least 0.7 times the diameter of the droplet. The yield stress value is the force which must be applied to the material in order to obtain flow thereof.

In the drawings:

FIG. 1 is a schematic side view showing apparatus which can be used for carrying out the method of the present invention;

FIG. 2 is a perspective view of a shape formed in accordance with the invention, and

FIG. 3 is a perspective view of a shape formed in accordance with a further embodiment of the invention; and

FIG. 4 is a perspective view of a shape formed in accordance with another embodiment of the invention.

Referring moe particularly to the drawings where there is seen a flowable composition 1 comprising a suitable fluid settable binding material and abrasive particles if desired contained in asuitable vessel 2. An agitator 3 can be used to maintain uniform mixing or the composition. The composition is carried through pipes or tubing 4 by pump 5 to .a manifold 6, from which it passes into a plurality of tubes 7 having orifices at the ends thereof. The composition is pumped at a speed sufiicient to cause individual droplets 8 to fall from the orifices. In the case where the orifices are circular in shape, droplets 8 assume a characteristic conical shape upon impact on an endless belt 9 of stainless steel, fiberglass, or the like. Belt 9 driven by motor 10 conveys the comically-shaped particles through a curing chamber 11 which may be an oven or drying chamber depending'on the nature of the binding material used. .The hardened shapes are then collected ina receiving receptacle 12. Shapes containing vitrifiable ceramic or clay-like binders are then fired, for example, in a rotary kiln while shapes containing resinous binding materials are ready for packaging.

Pump 5 should be of a type which is not a positive displacement type. Rather, it has been found that optimum droplet formation is achieved by the use of a pump which permits liquid to flow out more rapidly than the pumping rate asa drop is pulled away from the orifice by gravity. It is preferred to use a two roll viscosity pump, but other suitable pumps will be apparent to those skilled in the art.

The abrasive inorganic filler particles used in forming the media of the present invention may be finely divided material including such known types as alumina, silicon carbide, titanium carbide, zirconia, silica, metals (such as steel, zinc, or tin), chalk, or talc. It will be understood that for fast cutting rates the filler particles should be hard; while for fine cutting or polishing, soft particles such as metals, chalk, or talc are preferred. The particles of this material are often angular in shape to provide abrasiveness and preferably are in an average size range of 1 to 500 microns. To provide an abrasive or cutting quality to the media, it is preferred to use compositions consisting of 30 to 75 percent by weight inorganic abrasive particles.

' Various settable binding materials can be used; As

' noted above, vitrifiable ceramics such as clays or frit containing compositions are suitable in which case the ceramic and abrasive particles, if desired, are mixed with wateror a fugitive organic binder to the desired consistency prior to use. The shapes are dried after formation to harden them sufficiently to permit firing in a kiln. The shapes are generally fired to the softening point of at least some of the constituents of the ceramic composition in order to cause some degree of vitrification. Other suitable settable binders include the cementitious type such as portland cement which attain a permanent set on air curing. The inorganic compositions in themselves contain filler particles which provide the media with abrading orpolishing qualities, but if desired, harder abrasives can be added. The vitrified portion of the ceramic composition acts as a binder for the media.

Suitable resinous binding materials include polyester resins and preferably polyester-styrene copolymers, phenolics, epoxy resins, etc. Such resinous materials are generally mixed with the abrasive particles and a suitable catalyst shortly before drop formation and may be either self-hardening or hardening may be accomplished by passing the shapes through a heated oven, irradiating chamber or the like wherein the resinous constituent is polymerized or cross linked.

The settable binding materials useful in forming the shapes of the invention are herein defined as permanently settable. This term as used herein is intended to include thermosetting resins, vitrifiable ceramics, and moisture curable ceramics, all of which, after settingare insoluble in hot Water, even under alkaline or acidic conditions in a pHrange of about 1.5 to 12 and resist permanent deformation at pressures of at least about 50 psi.

It has been found that in order to form individual 4 drops which are shape-stable prior to curing or hardening of the binding material that the viscosity of the mixture measured at 2 rpm. on a rotating spindle viscometer must be between about 500 and about 30,000 poise. Since the materials involved are. thixotropic and/or pseudoplastic in nature, the viscosity readings measured over a longer period of time or at a higher r.p.m. will be lower. At the lower limit useful shapes are produced by mixtures having viscosities of approximately 500 poise by deposition through an orifice no larger than about 0.1" which is a practical lower size. limit for the shapes. If the viscosity of the mixture is too low, fiat button-like shapes will be produced which will wear excessively thus greatly increasing the cost of the tumbling media. On the other hand compositions having a viscosity greater than about 30,000 poise will not easily form droplets that can be deposited on a collecting surface such as a belt.

- Compositions having viscosities of this magnitude are useful only with orifices about 1.5" or more in diameter and will form only large shapes. Shapes having diameters as large as 4 inches or more are possible in accordance with the invention.

The wearing characteristics of the shapes are improved by lowering the surface area-to-volume ratio which can be done in the case of a cone by increasing the height-todiameter ratio. There are practical limits, however, as to how high a cone can be made by the process due to the tendency for the shape to fall over thus greatly reducing the uniformity of the shapes. Height-to-diameter ratios ranging to about 0.75 and about 1.5 have been found useful.

It will be understood that shape having other than a fiat bottom can be produced by the use of irregularly surfaced collecting belt. Irregularity. may be desirable in some cases in order to prevent sticking of the tumbling shapes to the walls of the tumbling drum by suction. Any described texture can be provided on the botton surface of theshapes by providing a belt surface which is quilted, ribbed, dimpled, or corrugated. A deeply textured surface is not desirable, however, since release problems from the belt can be created. By substantially fiat as the term is employed herein is meant a surface not having mold cavities which form more than one generally planar surface of the shapes.

Shapes other than conical can be formed by the use of orifices having configurations other than circular, for example, rectangular, triangular, elliptical, indented, or other orifice cross sections can be employed to form a variety of desirable shapes. For example, it has been found that the use of an orifice which is generally triangular but with indented sides will produce shapes of the general type shown in FIG. 3, having good properties for finishing internal surfaces such as holes and undercuts. Other nonconical shapes can be produced by inclining the surface of the belt to produce a shape such as that shown in FIG. 4 which has a more rounded top surface.

The following examples, in which all proportions are given in parts by weight unless otherwise indicated, will serve to illustrate, but not limit, the invention.

EXAMPLE I A pastelike composition was formed by mixing parts of a resin system comprising 36% styrene monomer and 64% unsaturated O-phthalic polyester, 6 parts by weight of 50% benzoyl peroxide paste catalyst, parts 200 mesh silica. Several portions of this mixture were made upcontaining varying amounts of finely divided fire-dry pyrogenic silica (available under the trade name Cab-O-Sil) to give compositions having varying viscosities. These compositions were slowly metered through inch, /2 inch, and inch I.D. tubes from 2-, 8- and 16-inch drop heights, onto a horizontal surface of an endless belt. The results of these experiments are set forth in Table 1 including the dimensions of the essentially conical shapes formed. The viscosities reported were meansured on a Brookfield RVF viscometer, No. 7 spindle, at 2 rpm and 22-23 C.

2. A process for producing abrasive tumbling media comprising:

(a) providing a fiowable thixotropic composition con TABLE I Parts finely divided silica Chip Drop Dimension, in. Height per 100 parts Viscosity Tube weight, height, diameter resin poise LD. g'rn. in. Diameter Height ratio Shape 35 2 38 53 Button. 0.9 2, 000 M 35 8 39 20 51 Do. 38 16 .42 15 36 Do. 1. 47 2 63 26 41 Button-top strings over. 0.9 2, 000 1. 44 8 67 28 42 Button. 1. 30 16 66 23 o. 3. 83 2 1. 00 26 26 Button-top strings over. 0.9 2, 000 3. 46 8 1. O6 31 29 Button. 3. 32 16 1.00 .25 25 Do.

. 96 2 34 (.22 x 1.36) Tear drop-fell over. 2.9 4, 500 90 8 47 85 Con cal.

1. 04 16 51 36 71 Do. 2. 52 2 (.40 x .8) Fell over. 2.9 4, 500 1. 86 8 63 41 65 Conical.

1. 88 16 37 53 Button. 5. 74 2 89 60 68 Top strings. 2.0 4, 500 5. 98 8 1. 00 57 57 Button. 6. 23 16 1. l3 49 43 D0. 1. 28 2 33 23 x 1 3) Fell over. 3.8 8, 500 1. 34 8 48 67 1. 39 Conical.

4. 47 2 60 42 x 1 5) Fell over. 3.8 8, 500 3. 81 8 77 95 Conical.

3. 83 16 85 60 79 D0. 9. 17 2 (.68 x 1.4) Fell over. 3.8, 8, 500 9. 00 8 1. 13 83 74 Conical. 8. 77 16 1. 19 66 55 Button.

EXAMPLE I sistlng essentially of about 30 to 75 percent abrasive 9.4 grams diethylene triamine 93.8 grams silicon carbide, 1000 mesh 76.8 grams liquid epoxy resin (epichlorohydrin-bisphenol a type having an epoxide equivalent of about 190).

The ingredients listed above were blended together and pumped through a /2 I.D. tube, vertically mounted about 8" from the bench. Drops falling formed conical shapes and after curing 30 minutes at F. were about .50" high by .69" diameter.

EXAMPLE III 25 gm. Tennessee Ball Clay #5 50 gm. flint, 200 mesh 25 gm. feldspar, 200 mesh 75 ml. water This blended mixture was allowed to drop from /z" LD. tube from a height of 2". The shapes were dried 16 hours at 150 F. and fired to cone 10 down in an electric kiln. White, very smooth, conical shapes resulted having a .43" height by .62" diameter.

What is claimed is:

1. A process for producing tumbling media comprising:

(a) providing a thixotropic flowable permanently setable composition consisting essentially of 30 to 75% by weight of inorganic filler particles and a settable fluid binder material selected from the group consisting of ceramics, portland cement and thermosetting resins said composition having a viscosity between about 500 and about 30,000 poise, measured on a rotating spindle viscometer at 2 r.p.m., the thixotropy of said composition being such that the composition has a yield stress of at least 15,000 dynes/cm. as measured on a Ferranti-Shirley plate and cone viscometer and suflicient to maintain a droplet of said composition supported on a flat level surface at a height at least 0.75 times the diameter of said droplet;

(b) depositing droplets of said composition on a substantially hard flat surface of a horizontally moving belt whereby the droplets form into individual hardenable shapes having a flat bottom and upwardly tapering side walls having a height about 0.75 to 1.5 times the diameters of the flat bottoms;

(c) hardening said composition; and

(d) collecting the hardened shapes from said surface.

inorganic particles having an average diameter of about 1 to 500 microns and a settable fluid binding material selected from the group consisting of ceramics, portland cement and thermosetting resins said composition having a viscosity between about 500 and about 30,000 poise, measured on a rotating spindle viscometer at 2 r.p.m., the thixotropy of said composition being such that the composition has a yield stress of at least 15,000 dynes/cm. as measured on a Ferranti-Shirley plate and cone viscometer and suflicient to maintain a droplet of said composition supported on a flat level surface at a height at least 0.75 times the diameter of said droplet;

(b) discharging said composition dropwise from an orifice having a maximum cross sectional dimension greater than about 0.1 inch onto a hard substantially flat surface of a horizontally moving belt whereby the droplets form into individual hardenable shapes having a flat bottom and upwardly tapering side wall having a height of about 0.75 to 1.5 times the diameter of the flat bottoms formed by impact of the droplets on said belt;

(d) conveying said shapes on said belt through a hardening zone to harden said shapes, and

(d) collecting said hardened shapes from said belt.

3. The method of claim 1 wherein said composition comprises a vitrifiable ceramic composition, said hardening is carried out by drying said ceramic composition, and said shapes are further hardened by firing in a kiln.

4. The method of claim 1 wherein said composition comprises a resinous polymer hardenable to resist a crushing force of at least 50 p.s.i. and said hardening is carried out by passing said shapes through a chamber containing an environment which Will accelerate polymerization of said material.

References Cited UNITED STATES PATENTS 2,872,719 2/ 1959 Brassfield et al. 264-56 2,947,124 8/1960 Madigan et al 51308 2,978,850 4/1961 Gleszer 51--307 3,102,011 8/ 1963 Bellinger 51298 DONALD J. ARNOLD, Primary Examiner US. Cl. X.R. 

